1. Field of Invention
This invention is related to interferometric optical devices, particularly to interferometric optical profilers used in topography measurement.
2. Description of Prior Art
An interferometric optical profiler is a device which uses optical interference to measure the profile of a sample surface without physical contact. Surface profile measurement is often required in semiconductor, data storage, and fiberoptic telecommunication industries. For example, inspection of a silicon wafer's surface is often performed in the semiconductor industry. There are two major types of interferometric optical profilers: imaging and scanning types.
An imaging type profiler produces an optical image of a surface area. It is usually based on Michelson, Mirau, Linnik, or Fizeau interferometers, which use a beam splitter to split a beam from a light source into two beams by amplitude division. The two beams travel along separate paths and are reflected by a tunable reference mirror and a sample surface, which are in different places. The reflected beams are then recombined by the beam splitter and interfere with each other. The interference results in interference patterns, or intensity distribution on a detector.
The interference patterns depend upon two factors: optical path length difference between the two paths and the sample's surface profile. Thus the surface profile can be obtained by utilizing interference patterns and the corresponding tunable optical path length difference. The measurement is fast, but is sensitive to vibration since vibration of either the reference mirror or the sample changes the optical path. Because of vibration, such surface profilers are difficult to use in a production environment, for example, on a production line.
A scanning type profiler scans a surface to collect topography data. It is usually based on a concentric-beam interferometer or a common-path polarization interferometer. Although a scanning type profiler creates and makes use of two beams as well, the beams travel on either the same optical path, or on side-by-side paths. As a result, vibration effects are reduced. But due to its scanning nature, a scanning type profiler has a slow measurement speed which limits its applications.
Accordingly, current interferometric optical profilers are unable to make rapid measurements with good vibration insensitivity.